You can also automate the process by creating an alert that changes the custom property, plus as a bonus feature you can leverage this to automate the future.
Simply create a new alert this a totally unique name, just so you can delete or edit it in the future. Given your example the scope of the alert would simply be Department is equal to AC - Ampersand Infrastructure and the trigger condition is the node is up.
You don't need a reset condition.
The trigger action will be Change Custom Property and in the pop-up window simply select Department as the custom property name and then select the new department name in the Custom Property Value drop down.
Save changes, click next x2 and submit to save.
If done correctly node(s) will be listed in the box above the submit button, and the alert will show in the 'alerts this object can trigger widget'.
Within a few minutes the alert will kick off and it'll change the custom properties for you.
